YOU SAID:
I forgot what butter is
INTO JAPANESE
私はバターが何かを忘れていた
BACK INTO ENGLISH
I had butter forgot something
INTO JAPANESE
私はバターを忘れてしまった
BACK INTO ENGLISH
I have forgotten butter
INTO JAPANESE
私はバターを忘れてしまった
BACK INTO ENGLISH
I have forgotten butter
Come on, you can do better than that.